1. Field of the Invention
The present invention relates to a transfer device and an image forming apparatus.
2. Description of the Related Art
Conventionally, there is known a transfer device of an image forming apparatus that forms a transfer nip by a nip forming member that abuts an image bearer, sandwiches the sheet that is a transfer material in the nip, and transfers a toner image on the image bearer onto the sheet. As one example, there is known a secondary transfer device that transfers a superimposed toner image formed on an intermediate transfer belt, onto a sheet. Specifically, first, toner images of different colors are formed on corresponding photoconductive drums, and the toner images are sequentially transferred, by primary transfer, onto the intermediate transfer belt, to form the superimposed toner image. The intermediate transfer belt is a belt image bearer that is rotatably stretched around a plurality of stretching members. Then, the superimposed toner image formed on the intermediate transfer belt is transferred, by secondary transfer, onto the sheet.
In the secondary transfer device, a nip forming member is provided. The nip forming member includes a secondary transfer roller that abuts the front side of the intermediate transfer belt, and a secondary transfer counter roller that is faces the secondary transfer roller and that abuts the back side 1 of the intermediate transfer belt. The secondary transfer counter roller is one of the plurality of stretching members. The secondary transfer roller and the secondary transfer counter roller sandwich the intermediate transfer belt to form a secondary transfer nip. The superimposed toner image formed on the intermediate transfer belt is transferred onto a sheet, which is conveyed to the secondary transfer nip.
In an image forming apparatus provided with the above secondary transfer device, when the movement speed of the intermediate transfer belt varies within one rotation period of the intermediate transfer belt due to variations in the thickness of the intermediate transfer belt, etc., the superimposition position may shift. The superimposition position is where the toner images of different colors are to be superimposed from the plurality of photoconductive drums. Thus, when the superimposition position is shifted, a failure such as color shift may occur.
An image forming apparatus described in Patent Document 1 is provided with a driving roller speed detector for detecting the rotational speed of a driving roller, which is one of the plurality of stretching members for drivingly rotating the intermediate transfer belt, and a belt speed detector for detecting the movement speed of the intermediate transfer belt. Based on the detection results of the driving roller speed detector and the belt speed detector, the rotational speed of the driving roller is adjusted by controlling a driving motor for driving the driving roller, such that the movement speed of the intermediate transfer belt becomes a constant speed.
However, in a conventional secondary transfer device, the rotating shaft of the secondary transfer roller is drivingly rotated at a constant speed by the motor. Therefore, when the roller diameter changes as the surface of the secondary transfer deforms, etc., the surface speed of the secondary transfer roller varies. Furthermore, the secondary transfer roller generates a belt conveying force by contacting the intermediate transfer belt, and therefore when the surface speed of the secondary transfer roller varies, the conveying force of the intermediate transfer belt, which is caused by the secondary transfer roller, also varies.
As described above, when the conveying force of the intermediate transfer belt by the secondary transfer roller varies, a phenomenon occurs, in which the belt part between the driving roller and the secondary transfer roller is pulled and loosened in the intermediate transfer belt rotation direction. When this phenomenon occurs significantly, even by controlling the rotational speed of the driving roller such that the movement speed of the intermediate transfer belt becomes constant, it is difficult to control the movement speed of the intermediate transfer belt with high precision.
That is, when the surface speed of the secondary transfer roller increases, and the conveying amount of the intermediate transfer belt by the secondary transfer roller increases, the intermediate transfer belt is pulled with respect to the driving roller and the belt tension increases, the conveying load on the driving roller is decreased, and the conveying torque of the driving roller is decreased. In this state, even by slightly increasing the turning force of the driving roller, the conveying amount of the intermediate transfer belt becomes excessively large. Accordingly, it becomes difficult to finely adjust the movement speed of the intermediate transfer belt, and it becomes difficult to control the movement speed of the intermediate transfer belt with high precision by controlling the rotational speed of the driving roller.
Meanwhile, when the surface speed of the secondary transfer roller decreases, and the conveying amount of the intermediate transfer belt decreases, the intermediate transfer belt is loosened with respect to the driving roller and the belt tension decreases, and the conveying load on the driver roller increases, and the conveying torque of the driving roller increases. In this state, when the conveying torque of the driving torque extremely increases, slipping is caused between the driving roller surface and the intermediate transfer belt, and the conveying amount of the intermediate transfer belt by the driving roller decreases. Therefore, a conveying failure of the intermediate transfer belt by the driving roller occurs, and therefore it becomes difficult to control the movement speed of the intermediate transfer belt with high precision by controlling the rotational speed of the driving roller.
As described above, even when the rotational speed of the driving roller is controlled such that the movement speed of the intermediate transfer belt becomes constant, due to the impact of the variations in the surface speed of the secondary transfer roller, the movement speed of the intermediate transfer belt cannot be controlled with high precision.
In the image forming apparatus described in Patent Document 1, when the current value of the driving motor detected by a current value detecting unit is lower than a target current value of a predetermined range, in the state where the secondary transfer roller and the intermediate transfer belt are in contact, it is determined that drag turning of the intermediate transfer belt has occurred. In order to resolve the drag turning, control is implemented to decrease the rotational speed of the secondary transfer roller such that the current value is included in the predetermined range of the target current value, and the set speed of the secondary transfer roller is adjusted.
However, the target current value of a predetermined range is set according to design parameters such as the motor specification and the control margin, and the target current value is set to have a wide range in consideration of the individual differences (tolerance), the environment, and time-dependent variations. Therefore, by adjusting the set speed of the secondary transfer roller based on a wide predetermined range, such that the current value of the driving motor detected by the current value detecting unit is lower than the target current value of a predetermined range, the adjustment is insufficient, and the image quality demanded by the market cannot be obtained.
Furthermore, there is known an image forming apparatus of a direct transfer method, in which a toner image on a photoconductive belt that is a belt image bearer, is directly transferred onto a transfer material such as a sheet, at a transfer part formed by causing the photoconductive belt and the transfer roller to contact each other. Also in such an image forming apparatus of a direct transfer method, according to the same reasons as above, it is difficult to control the movement speed of the photoconductive belt with high precision due to the impact of the variations in the surface speed of the transfer roller.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2011-180565